How to read a TAF

What an aerodrome forecast says, how BECMG, TEMPO and PROB groups work, and how to use it when planning a flight.

The short answer

A TAF is a forecast for one aerodrome, usually covering 9, 24 or 30 hours. It uses the same codes as a METAR, plus change groups that tell you when and how the weather is expected to change.

The validity period

After the station and issue time comes the validity, for example 2412/2512: from the 24th at 12:00 UTC until the 25th at 12:00 UTC. The base forecast that follows is what the forecaster expects for most of that period.

Change groups

• BECMG 2415/2417: conditions change gradually between 15:00 and 17:00 UTC and then stay
• TEMPO 2418/2422: temporary fluctuations, each lasting less than an hour and together less than half the period
• FM241800: from 18:00 UTC a new set of conditions replaces the old one completely
• PROB30 or PROB40: a 30 or 40 percent chance of the conditions that follow

Using it for planning

Plan against the worst conditions forecast for the time you will be there, including TEMPO and PROB groups. For destination and alternate choices, operators often apply a buffer of an hour either side of your expected arrival time.